1. Field of the Invention
The invention relates to the data transfer control systems, and more particularly to a method and system for maintaining a data transfer rate through a peripheral controller to a peripheral storage device without the loss of data.
2. Prior Art
Data processing systems having a plurality of system units electrically coupled to a common communication bus for the asynchronous transfer of information therebetween are disclosed in U.S. Pat. No. 3,993,981 and in U.S. application Ser. No. 643,439 filed Dec. 22, 1975, each assigned to the assignee of the present invention.
In the transfer of data from a main memory unit of such a data processing system to a peripheral storage device, two problems may occur which cause a degeneration of the data transfer rate. If the peripheral controller does not request an additional data word from main memory immediately upon receiving a data word in response to a previous request, the communication bus may be captured by another system unit. Further, if the peripheral controller requests data at a rate exceeding the transfer rate to the peripheral storage device, data may be lost.
In prior systems, the data transfer rate has been decreased below a safety threshold to avoid the above-mentioned problems, or data requests have been issued immediately upon receipt of a data word in response to a previous request without regard to the availability of storage locations. In operating environments where the data transfer rates approach the marginal areas of safety, neither of these approaches have proven satisfactory.
The present invention provides a method and system for predicting the storage capacity of a peripheral controller before a data word request is issued. Access to the common communication bus thereby is maintained as required to accommodate the data transfer rate, and no data is lost.